The present invention relates to a field-effect transistor, and a semiconductor device and photo relay including the field-effect transistor and, more particularly, to an analog switch (relay or antenna switch) and photo relay for controlling a signal line, a power transistor for handling energy, and the like.
Although the conventionally extensively used mechanical contact type relay has excellent high-frequency characteristics, demands for replacing this relay with a photo relay using a field-effect transistor are increasing. For this purpose, it is necessary to reduce an ON resistance Ron and OFF capacitance Cout of the field-effect transistor.
Unfortunately, the ON resistance Ron and OFF capacitance Cout of the field-effect transistor have tradeoff.
It is difficult to reduce both the ON resistance Ron and OFF capacitance Cout of the conventional field-effect transistor, and Cout×Ron>10 [pF·Ω] is the limit.
Reportedly, it is possible to increase the channel mobility of the field-effect transistor, which is used as an information processing transistor having a breakdown voltage of a few volts in an LSI or the like, by using an intentionally strained silicon layer, i.e., a strained silicon layer. However, a thickness by which this strain stably exists is usually a few nm to a few tens of nm. Accordingly, it is regarded as impossible to apply this transistor to, e.g., an analog switch (relay), photo relay, or power transistor required to have an element breakdown voltage of a few tens of volts or more.
A reference disclosing a technique concerning the conventional semiconductor relay is as follows.    1: Japanese Patent Laid-Open No. 9-312392
Also, a reference disclosing a technique concerning a photo relay having the conventional SOI structure is as follows.    2: Japanese Patent Laid-Open No. 11-186562
As described above, although the reliability of the mechanical contact type relay is inferior to that of the semiconductor relay, it is difficult to obtain a low ON resistance Ron and small OFF capacitance Cout which are sufficient to replace the mechanical contact type relay, and ensure a desired element breakdown voltage.